New message helper to replace methods on TestCase subclasses.
Update all the vumi.application tests to use the new helper.
Update all the vumi.components tests to use the new helper.
Update all the vumi.dispatchers tests to use the new helper.
Update all the vumi.tests tests to use the new helper.
Update all the vumi.demos tests to use the new helper.
Update all the vumi.transports tests to use the new helper.